Detection apparatus

ABSTRACT

An ion mobility spectrometer has a pair of electrodes ( 13 A) and ( 13 B) midway along the drift chamber ( 7 ). A high field is applied between the electrodes ( 13 A) and ( 13 B) sufficient to modify (e.g. fragment) ions in the region of the electrodes such that they move at a different rate towards the collector plate ( 8 ) . This is used to modify the time of flight of selected ions or ion clusters and enable identification of ambiguous peaks on the IMS spectrum.

This invention relates to detection apparatus of the kind for detectingsubstances in a sample.

The invention is more particularly, but not exclusively concerned withion mobility spectrometers (IMSs).

Ion mobility spectrometers (IMSs) are used to detect the presence ofsmall quantities of airborne chemicals in vapours or gases atatmospheric pressure. An IMS has some means to ionize the samplechemicals, such as a corona discharge or a radioactive source. A gate isopened to admit the molecular ion clusters into one end of a driftchamber across which a voltage is applied to cause the ion clusters todrift to the opposite end where they are collected on a collector plate.The molecular ion clusters might also include attached dopant ions. Thetime taken for a molecular ion cluster to pass from the gate to thecollector plate is dependent on the mass, size, shape and charge on themolecular ion cluster. By measuring this time an indication can beprovided of the nature of the chemical.

In many cases it can be difficult to identify positively the substanceof interest because the time of flight of the ion clusters produced maybe very similar to that of ion clusters of different substances. Variousarrangements have been proposed for improving the discrimination betweendifferent molecular ion clusters. One arrangement described in U.S. Pat.No. 6,797,943 involves fragmenting the ions using laser energy, apyrolyzer or the like. The ion clusters are accumulated in a reservoirwhere they are exposed to the ion modification energy prior toadmittance to the drift chamber. One problem with this arrangement isthat all the molecular ion clusters are subjected to fragmentation,which can lead to a large number of peaks on the spectrum, makinganalysis very difficult. Another problem with this arrangement is that arelatively large amount of energy is needed to ensure that fragmentationtakes place, which can be a particular problem in portable,battery-powered apparatus.

It is an object of the present invention to provide alternativedetection apparatus.

According to one aspect of the present invention there is provideddetection apparatus of the above-specified kind, characterised in thatthe apparatus includes an electrical arrangement for subjectingmolecules of the substances to an electrical field high enough to causeion modification of some at least of the molecules.

The apparatus preferably includes an ionizer for ionizing the sample andan arrangement for measuring the mobility of the ions produced, theelectrical arrangement causing ion modification of some at least of theionized molecules. The electric field is preferably symmetrical andcould be an RF field. The electric field could be applied in shortbursts. The molecules may be individual molecules or clusters ofmolecules and may include attached dopant molecules. The electricalfield may be arranged to cause modification of only selected ones of themolecules.

According to another aspect of the present invention there is provideddetection apparatus for detecting substances in a sample, characterisedin that the apparatus includes an arrangement for applying energy tomolecules of the substance sufficient to cause ion modification of onlyselected ones of the molecules.

According to a further aspect of the present invention there is provideddetection apparatus for detecting substances in a sample, characterisedin that the apparatus is arranged to separate out molecules of differentsubstances from one another into different regions of the apparatus andthat the apparatus includes an arrangement for applying energy to aselected one of the regions sufficient to cause ion modification of onlythose molecules in that region.

According to a fourth aspect of the present invention there is providedion mobility spectrometer apparatus including an ionization region, agate, a drift chamber and a collector, characterised in that theapparatus includes a pair of electrodes spaced from one another acrossthe apparatus, and that the apparatus is arranged to apply a high fieldbetween the electrodes sufficient to modify ions such that the modifiedions travel at a different speed to the collector from unmodified ions.

The pair of electrodes may be located in the drift chamber. Theapparatus may be arranged to apply the high field between the electrodesa predetermined time after opening the gate so that only ions with aselected range of mobilities are modified by the high field.

According to a fifth aspect of the present invention there is provided amethod of detecting substances in a sample including the steps ofapplying energy to modify only selected molecules of the substancesprior to detection.

The energy may be applied at times selected preferentially to cause ionmodification of ions having selected ranges of mobility. The energy ispreferably applied for short periods of time and/or at high frequency.The energy is preferably applied by means of an electrical field.

A time-of-flight ion mobility spectrometer according to the presentinvention will now be described, by way of example, with reference tothe accompanying drawing, which shows the spectrometer schematically.

The spectrometer includes an elongate, tubular housing 1 the interior ofwhich is substantially at atmospheric pressure. An inlet port 2 towardsits left-hand end opens into an ionisation region 3. A sample gas orvapour to be analysed is supplied to the port 2 via a filter 4, in theconventional manner. The ionisation region 3 includes a corona dischargepoint 5 or some other means, such as a radioactive source, for ionisingthe sample. A gate 6 separates the ionisation region 3 from a driftchamber 7, which extends to the right-hand end of the housing 1 where anion collector plate 8 is mounted. The drift chamber 7 includes a row ofelectrodes 11 to 15 spaced from one another along the length of thedrift chamber 7 in the usual way. The collector plate 8, electrodes 11to 15, gate 6 and discharge point 5 are electrically connected to aprocessor control unit 20, which provides an output to a display orother utilisation unit 21 representative of the substances detected.

Drift gas is supplied to the right-hand end of the housing 1 at an inlet30 to flow from right to left along the drift chamber 7, that is, in theopposite direction from the flow of ions. Drift gas is exhausted fromthe drift chamber 7 at its left-hand end through an outlet port 31. Gasflows between the outlet port 31 and the inlet 30 via a gas system 32,which includes a molecular sieve 33 and a pump 34. The sieve 33 maycontain a dopant in the manner described in WO00/79261. Ions produced inthe ionisation region 3 are admitted into the left-hand end of the driftchamber 7 when the gate 6 is opened. The ions drift from left to rightalong the drift chamber under the influence of the relatively lowelectrical field of about 250 V cm⁻¹ applied by the electrodes 11 to 15.The ions of different mobilities separate out from one another as theypass along the drift chamber 7 so that, at any one time, different ionswill be in different regions of the chamber. The ions of differentmobilities, therefore, reach the collector plate 8 at different timesand produce output peaks to the processing unit 20 at different times.

As so far described, the apparatus is conventional.

The apparatus differs from previous apparatus by the inclusion of meansto apply a high electrical field to cause ion modification of the ions,such as fragmentation. There are various ways in which this could beachieved. The high field could be applied at any part of the apparatus,but, in the present example, it is applied in the drift chamber 7. Thefield is applied by means of the electrode 13, which is separated intotwo electrodes 13A and 13B spaced from one another across the diameterof the drift chamber 7. Although these electrodes 13A and 13B are shownmidway along the drift chamber 7, they could be located at any pointalong the chamber. These electrodes 13A and 13B are connected to a highvoltage RF unit 40 controlled by the processing unit 20. The highvoltage unit 40 is operable to apply a high strength RF field (typicallyaround 2 MHz) effective to cause ion modification of a significantpercentage of the ions within the field. The strength of the field ispreferably at least 10,000 V/cm and may be of the order of several tensof thousands of volts per centimetre. The RF field may be appliedcontinuously or in bursts of the order of 1 μs in order to preventcorona discharge. The symmetrical nature of the applied field isadvantageous because it ensures that the molecular ion clusters remainsubstantially central within the drift chamber 7 and are not displacedto contact with the electrodes 13 to 15. Alternatively, non-RF intenseshort pulses of the order of 1 ns could be used. These fields enablesufficient energy to be transferred to the ions to cause ionmodification. The modified or fragmented ions pass to the collectorplate 8 with a different mobility from the unmodified ions and henceproduce different peaks on the output spectrum. This can enable theapparatus to distinguish between two different ions having similarmobilities, since the modified versions of these ions will not generallyhave similar mobilities.

The sample gas or vapour could include a dopant selected to stabilisethe neutral molecule environment in the device, such as water at a fewhundred ppm.

The apparatus could be arranged to operate conventionally most of thetime with the two electrodes 13A and 13B being at the same dc voltage,between the voltages on adjacent electrodes 12 and 14, that is, withoutthe ion modification field. When an ambiguous substance is identified,the processing unit 20 would initiate the ion modification field inorder to resolve the ambiguity. Alternatively, the apparatus could beoperated with the ion modification field continuously on and then turnit off for short periods to confirm detection of a substance.

Application of the ion modification field could be coupled to operationof the gate 6. In such an arrangement, the ion modification field isinitiated at a calculated predetermined time after opening the gate 6such that only those ions within a selected range of mobilities are inthe region of the ion modification field electrodes 13A and 13B whenthey are energized with the ion modification voltage. This arrangementhelps confine the ion modification process to selected ions only,thereby avoiding too many peaks being produced on the spectrum andfacilitating identification.

Alternative energy sources could be used to produce ion modification onselected ionized molecules in a similar manner. Such alternative energysources could include: radiation sources, such as laser radiation, UVradiation, VUV radiation, infra-red or photo ionisation, electronbombardment, electron beams, electro spray, electron-ionisation, coronadischarge, glow discharge, plasma or radioactive emission.

The ion modification field could be applied at any location, such as inthe ionisation region 3 or at the gate 6 and could be applied at anytime during each scan of the IMS apparatus. The invention could beapplied to detection apparatus other than IMS apparatus.

1. A detection apparatus for detecting substances in a sample,comprising: an ionizer for ionizing the sample and an electricalarrangement for applying an electrical field to ions of a substance,included in the sample, sufficient to cause ion modification of at leastsome of the ions, wherein the ion modification occurs in a drift chamberand alters the at least some of the ions' ion mobility.
 2. The apparatusaccording to claim 1, further comprising an arrangement for measuringthe mobility of the ions produced.
 3. The apparatus according to claim1, wherein the electrical field is symmetrical.
 4. The apparatusaccording to claim 1, wherein the electrical field is an RF field. 5.The apparatus according to claim 1, wherein the electrical field isapplied in short bursts.
 6. The apparatus according to claim 1, whereinthe ions are individual molecules or clusters of ions.
 7. The apparatusaccording to claim 1, wherein ions include attached dopant molecules. 8.The apparatus according to claim 1, wherein the electrical field isarranged to cause ion modification of only selected ones of the ions. 9.The detection apparatus according to claim 1, wherein the ionmodification comprises fragmentation.
 10. A detection apparatus fordetecting substances in a sample, comprising an arrangement for applyingan electrical field to ions of the substance sufficient to cause ionmodification of only selected ones of the ions, wherein saidmodification occurs in a drift chamber.
 11. A detection apparatus fordetecting substances in a sample, comprising an arrangement for applyingan electrical field in a drift chamber sufficient to cause ionmodification of selected ones of ions in the drift chamber, and whereinthe apparatus is arranged to separate ions of different substances fromone another into different regions of the apparatus.
 12. An ion mobilityspectrometer apparatus comprising: an ionization region; a gate; a driftchamber; a collector; and a pair of electrodes spaced from one anotheracross the apparatus, wherein the apparatus is arranged to apply a highfield between the electrodes sufficient to modify ions in the driftchamber so the modified ions travel to the collector at a differentspeed from unmodified ions.
 13. The ion mobility spectrometer apparatusaccording to claim 12, wherein the apparatus is arranged to apply thehigh field between the electrodes at a predetermined time after openingthe gate so that only ions with a selected range of mobilities aremodified by the high field.
 14. A method of detecting substances in asample comprising: (a) ionizing molecules in the sample; (b) applying anelectrical field to modify selected ions of the substances prior todetection, wherein the modification occurs in a drift chamber and altersthe selected ions' ion mobility; and (c) detecting the selected ionsbased on their mobility.
 15. The method according to claim 14, whereinthe electrical field is applied at a time selected to preferentiallymodify the selected ions' ion mobility.
 16. The method according toclaim 14, wherein the electrical field is applied for short periods oftime so that only ions within a selected range of mobilities areenergized by a modification voltage of the electric field.
 17. Themethod according to claim 14, wherein the energy is applied at highfrequency so that only ions within a selected range of mobilities areenergized by a modification voltage of the electric field.
 18. Themethod according to claim 14, wherein the energy is applied by means ofan electrical field.
 19. An ion mobility spectrometer apparatuscomprising: (a) an ionization region; (b) a gate; (c) a drift chamber;(d) a collector; and (e) a pair of electrodes spaced from one anotheracross the apparatus and located in the draft chamber, wherein theapparatus is arranged to apply a high field between the electrodessufficient to modify ions such that the modified ions travel at adifferent speed to the collector from unmodified ions.